In many cases a bishop pair is stronger than a bishop and a knight or two knights.
The question is, if both sides have already traded one of their bishops, losing the bishop pairs, should the other ones be traded as well?
First, note that
It is well-known that a bishop pair is stronger than a bishop and a knight or two knights.
is wrong, strictly speaking. Wheter either of these combinations is strongest depends on features of the position at hand, mainly the pawn structure, with pawn islands and outpost squares as its main aspects.
Second, note that trading without a gain, a clear reason why, is usually bad. Trade if there is a concrete reason, otherwise don't. Concrete reason includes longterm assets such as weak colour complexes.
In open or semi-open positions a bishop pair is often murder, and a lone bishop is usually superior to a lone knight. The knight is superior to the bishop in blocked positions or when the bishop is hemmed in by pawns on the same colour squares as the bishop.
In general, you should trade either of your pieces against your opponents remaining bishop if they have a weak color-complex thats currently defended by their bishop, even at the cost of trading your bishop, no matter what case, and you should not allow your opponent to trade either of their pieces for your remaining bishop if you have a weak color-complex yourself.
A special treatment is deserved when both sides have opposite coloured bishops; here, the initiative is played a piece up. Thus, when you're defending, trade for their bishop if possible, and refrain from trading if you have the iniative.
In short: Don't trade unless there is a shortterm gain or a longterm asset to play against, and let the pawn structure be the deciding factor.